In the present investigation we administered tetrahydrobiopterin (THB) and analyzed its ability to penetrate the blood-brain barrier. THB was injected at a dose of 15-20 mg/kg into rhesus macaques kept in a restraining chair, with CSF collected by continuous sampling through cervical cannulas. THB in CSF increased in the first 90 minutes and peaked at 3-4 hours following peripheral administration. Peak levels ranged from 3-30 nmoles/ml of CSF which represents as much as a 700-fold increase over baseline. Preliminary analysis of CSF monoamine metabolites suggests minimal changes. Further similar experiments are being performed using 6-methyl tetrahydropterin.